The present invention is directed generally to muting a radio channel in a radiocommunications system and, more particularly, to methods and apparatus for selectively squelching a radio channel based on a detection of in-band noise.
The first cellular mobile radio systems in public use were analog systems for conveyance of speech or other analog information. These systems utilized multiple radio channels to communicate analog information between base and mobile stations by transmitting analog-modulated radio signals. One such system is known as the Nordic Mobile Telephone system NMT 450. Another known analog cellular mobile radio system, of particular interest as background to the present invention, is the Advanced Mobile Phone Service (AMPS) mobile radio system utilized in the U.S.A. In general, these systems provide a squelching system which relies on the detection of noise in order to determine whether squelching is to be applied.
Recently, digital cellular mobile radio systems for public use have been designed. Digital cellular mobile radio systems provide digital radio channels for transmitting digital or digitized analog information between base and mobile stations using digitally modulated radio signals. Digital cellular mobile radio systems may offer substantial advantages, e.g. greater system capacity per unit bandwidth, over analog cellular mobile radio systems.
In contrast to the introduction of digital-only cellular mobile radio systems, like the GSM system employed in parts of Europe, in areas with existing analog cellular systems it has been proposed to introduce digital cellular mobile radio systems which are designed to cooperate with the existing analog cellular mobile radio systems. In this way large legacy customer bases will not suddenly find that their terminal equipment has become obsolete. System designers of these hybrid systems believe that the digital portion of the system can be gradually introduced and, over time, the number of digital channels can be gradually increased, while the number of analog channels is gradually decreased. In order to provide complete compatibility, such dual-mode systems should comport with adopted standards for both analog and digital formats, such as the AMPS standard.
In cellular mobile radio systems it is desirable to provide a communications channel that is generally free of noise or interference, or in the event that such noise or interference is found, to mute the radio channel to enhance the perceived audio quality of a signal transmitted over the radio channel. It is also desirous that a mobile station with an established connection on a radio channel be able to maintain audio quality when moving within the same cell, or between cells, even if the radio channel being used is subject to increased interference. If the quality of the established connection falls below specified parameters it is further desirable to automatically mute, or squelch the radio channel momentarily, and then to restore the transmitted signal when the quality of the connection returns to an acceptable level. This practice avoids the perception of diminished audio quality.
The AMPS system provides a noise squelching technique whereby a radio channel is momentarily muted when a substantial amount of out-of-band noise is detected, and then restores the radio signal when the out-of-band noise level drops below a predetermined threshold. A voice, or information signal, normally occupies the 0-3 kHz frequency band (the "in-band" frequency range). The AMPS system monitors a higher (out-band) frequency range, in accordance with a known principle, wherein the detection of a sufficient amount of noise energy in this higher frequency range is believed to be indicative of noise, or interference in the in-band radio channel (hereinafter referred to as the "voice channel"). More specifically, the aforementioned conventional system operates in accordance with a known principle wherein a measurement of noise energy in the 17 kHz to 71 kHz bandwidth is considered determinative of whether there is noise or interference in the 0-3 kHz in-band voice channel. If a sufficient amount of noise energy is detected in the 17 kHz-71 kHz frequency band, it is assumed that voice channel interference is present, and that the voice channel should be squelched to preserve perceived audio quality.
Although dual-mode base stations and mobile stations continue to support analog system functions, such as squelching, the techniques by which these functions are supported are continuously being improved to reduce costs and improve quality. With the increased power of digital signal processors (DSPs), system designers are interested in implementing many signal processing techniques previously implemented using analog hardware components as DSP routines. DSP implementation has the added attraction of reducing the number of components, and hence the size, of base stations and mobile stations.
Of course, digital signal processing also has its limitations. One design tradeoff which system designers confront when trying to implement analog signal processing techniques as DSP routines is that of speed of execution of a routine versus the amount of DSP resources, e.g., the number of millions of instructions per second (MIPS), which are allocated to execute a given routine. Since digital signal processors are not yet sufficiently inexpensive or quick that the number of MIPS used for each routine are insignificant, system designers are called upon to develop innovative digital signal processing techniques that reduce the number of MIPS used so that a digital signal processor can handle as many tasks as possible.
For instance, the aforementioned squelching function in the analog domain examines noise energy in a 17 kHz-71 kHz range. However, a conventional DSP would have to devote a large amount of resources, i.e., MIPS, to the evaluation of signals in this range due, in part, to the high sampling rate required to sample such signals. For example, at least 80 MIPS would typically be required for processing signals in this frequency range.
Furthermore, Applicants recognize that signal energies present in the 17 kHz-71 kHz range may not necessarily be indicative of noise or interference. Non-noise co-channel signals, adjacent, or side band signals also may operate in, and consequently contribute energy to, the 17 kHz-71 kHz frequency band. Detection of energy from such signals may result in a false indication of noise, and the inadvertent invocation of voice channel squelching.
Consequently, direct application of digital signal processing to perform conventional noise squelching in radiocommunication systems is susceptible to erroneous noise indications. Furthermore, inordinate processing resources are required to sample the frequency band traditionally investigated for indications of in-band noise. Accordingly, new techniques are desirable for determining when to squelch a radio channel in radiocommunication systems.